The increase in the number of digital multimedia devices in residences has led to the appearance of domestic networks assuring the simultaneous distribution and at a high bitrate of data streams, such as High Definition (HD) video data streams, audio data streams or Internet data streams, these data being available at all or almost all points of the residence. Such a network, for which the structure is dependent on the distribution of rooms within the building (apartment, house with or without a second floor, etc.) in which it is installed, can be deployed using different technologies such as cable, Power Line Carrier (PLC), optical fibre or WiFi type wireless devices complying with the standards 802.11a/b/g or 11n. This latter standard authorizes the use of MIMO (Multiple Input Multiple Output) technology that is a multi-antenna technique enabling transmission efficiency to be improved in terms of bitrate and robustness in environments dominated by interferences.
These new solutions based on the standard 802.11n and the MIMO technologies each however have their own specificity according to the MIMO technique used. What follows is a reminder of the fundamental principles of MIMO technology.
The principle of MIMO systems is to transmit and/or receive signals via several transmission channels in order to obtain independent signals and increase the probability that at least one of the signals is not or is only slightly affected by fading. MIMO topologies can be broken down into two main categories:                systems called open loop systems, in which the transmission is implemented without previous knowledge of the propagation channel, these systems use Spatial Multiplexing (also called MIMO Matrix B) techniques or Space Time Block Coding (also called MIMO Matrix A) techniques, and        systems called closed loop systems, using the Beamforming technique and comprising a return channel to transmit, to the transmitter, information relating to the transmission channel.        
To summarize, spatial multiplexing (MIMO Matrix B) consists in dividing the stream of data to be transmitted into a plurality of elementary streams and in transmitting each of these elementary streams with a radio channel and a specific antenna. In the presence of multiple paths, as is the case in an indoor environment, the different elementary streams attain the receiver with a different spatial signature, thus enabling them to be easily differentiated. In an ideal environment and over short distances, this technique multiplies the physical bitrate and consequently the overall transmission capacity of the system. For longer distances or in more difficult conditions for transmission (poor signal to noise ratio), the bitrate deteriorates rapidly. This deterioration being essentially due to the fact that the transmitter has no information on the state of the transmission channel. The transmission cannot thus be adapted in accordance to it.
The other open loop technique, space time block coding (MIMO Matrix B), that is also blind concerning the state of the transmission channel, gives priority to robustness and diversity is contrast to the previously cited spatial multiplexing. The stream of data to be transmitted is replicated and transmitted, after coding, via several radio channels and associated antennas. Each of these streams is encoded via a known spatio-temporal coding algorithm. This technique is often used to increase the coverage zone of the system. As for the spatial multiplexing, this technique does not provide for a return channel in order to receive information on the transmission channel. The transmission cannot thus be adapted in accordance to it.
Finally the beamforming technique is employed in order to control the direction and the form of the radiation pattern of the transmission antenna, in order to thus increase the power in the selected transmission direction. This technique improves the transmission in the selected direction and the resistance to interferences and noise. The transmission capacity of the system can be increased in this way. Beamforming in transmission is theoretically the optimal means for transmitting via a multiple antenna system. This technique provided with channel information procures a gain of 6 dB for 2 antennas (or 12 dB for 4 antennas). In addition, it can be combined with other techniques such as spatial multiplexing and thus leads to very high transmission capacities.
The configurations according to the 802.11n standard that are currently most common on the market are of 2×2:2 type (2 transmission channels, 2 reception channels and 2 spatial streams), 2×3:2 or 3×3:2. The majority of these systems do not have a return channel required for the implementation of the beamforming technique. In reception, it is known to use an item of information called CSI (Channel State Information) in order to determine the state of the transmission channel and to demodulate the symbols received according to it. The CSI information is generally obtained by means of training symbols transmitted with the useful symbols. For the transmission, a return channel will be required in order to transmit an item of CSIT (Channel State Information at Transmitter) information to the transmitter in order to adapt the signals transmitted to the transmission conditions. But, this return channel is not implemented in MIMO technology due to the complexity of the implementation of this return channel.
Currently, some systems implement therefore the technique called “implicit beamforming” defined in the standard 802.11n. According to this technique, it is considered that the transmission channel is reciprocal. For example, if a system comprising a central terminal communicating with a client terminal is considered, the central terminal calculates the direction matrix of beams without the addition of information from the client terminal. It does this by estimating the uplink channel (client terminal to central server), for example by analysing the preamble of signals received. The central terminal then transmits signals to the client terminal considering that the downlink channel is reciprocal (i.e. identical or almost identical to the uplink channel). This reciprocal hypothesis is not valid in an environment dominated by interferences and rich in multiple paths corresponding to a domestic environment.
A beamforming type solution at transmission would be more appropriate for such environments. But as indicated previously, its implementation would lead to a significant increase on the processing part of the signal in the transmitter due to the great number of MIMO antennas and radio channels associated. Such a solution would be complex to implement, energy consuming and costly.
One purpose of the invention is to overcome the disadvantages described above.